Locard's Exchange Principle

Edmond Locard was a pioneer in the field of forensic science, and became known as the "Sherlock Holmes of France."  While that may seem like a bit of a back-handed compliment, like being called the "Wayne Gretzky of India" or the "Eddie Van Halen of Amish Country", Locard did indeed develop the very foundation on which modern crime scene investigation is constructed.  The arrogantly-named Locard's Exchange Principle states, "with contact between two items, there will be an exchange."  In basic terms, this means that whenever an individual (most commonly the perpetrator of a crime) interacts with the crime scene or the victim, there will be an exchange of evidence between the perpetrator and the scene/victim.

The following story is a work of fiction.  Any resemblance to psychos living or dead is purely coincidental.  No, really.  I promise.

It was ten-fifteen, on a chilly evening in downtown Boston.  Jennifer Reynolds and two of her girlfriends left the AMC Theater after thoroughly enjoying Sex and the City 2 for the fourth time.  One of the friends suggested that the three of them get something to eat, but Jennifer said no, she had to get up early for work in the morning, but the other two should go on ahead.  This was an unfortunate decision on Jennifer's part because if she had gone with her friends to grab a bite, she would've enjoyed a tasty hamburger and a strawberry milkshake instead of what she ended up getting, which was murdered.

As Jennifer walked, alone, to the parking garage across the street, she was followed by the notorious serial killer The Watertown Whacko (cue haunting violin music).  Just as Jennifer reached out to open the driver's side door to her 2005 Ford Mustang, the Whacko clubbed her over the head with a sawed off Louisville Slugger, Rico Petrocelli model.  Having rendered poor Jennifer unconscious, the Whacko then strangled her to death with a length of rope and violated her corpse repeatedly.  When the deed was done, he fled the scene and went home where he put on a pair of his mother's pantyhose, slathered himself in Fluffernutter, and watched reruns of the The Three Stooges while pleasuring himself.

You don't pick up a nickname like "The Watertown Whacko" by being polite and charming.

Which brings us back to Locard and his exchange principle.

The detectives and forensic team investigating the Jennifer Reynolds murder would undoubtedly discover a great deal of physical evidence.  The Watertown Whacko is a disorganized killer, a man of opportunity rather than a meticulous planner.  Among other things, meticulous planners do not, as a rule, slather themselves in Fluffernutter because it's a sticky substance that's difficult to wash off.  They opt instead for a slicker lubricant, perhaps margarine, maybe vegetable oil.

Anyway, because the Whacko is careless, he exchanged a good deal of evidence with Jennifer.  He left semen (unless he used a condom, but let's face it, he probably didn't), hair, fibers from his clothing, spittle, and other microscopic DNA evidence on the bludgeoned and strangled carcass.  Conversely, he also picked up evidence -- strands of Jennifer's blond hair and other fibers, in addition to matted blood on the Rico Petrocelli baseball bat and microscopic DNA on the rope.

The Whacko also exchanged evidence with the crime scene itself, leaving fingerprints on Jennifer's Mustang, footprints on the floor of the parking garage, and perhaps picking up pebbles and dirt in the treads of his sneakers.

All of this evidence would be collected and analyzed by law enforcement officers, and hopefully the Watertown Whacko could be identified, located, and brought to justice.  I'm thinking "death by flaying" would not be too extreme a punishment.

So that's Locard's Exchange Principle in a somewhat over-sized nutshell.  But it makes me wonder, could this principle be applied in areas outside of forensic science?

Every day, we come in contact with hundreds of other people.  I don't mean physical contact, necessarily, I'm talking about conversations, verbal exchanges, even brief smiles or quizzical glances while in line at the grocery store.  Is it possible to encounter another human being and NOT have some sort of exchange take place?

A few weeks ago, I was at a local Subway restaurant.  I ordered my usual, the Italian BMT, and when I got to the register to pay, the cashier told me that it was covered, the lady in front of me had taken care of it.  By the time I turned around to thank her, she had already left.  Since I already had my credit card out, I figured I'd continue the gesture by paying for the meatball sub the guy behind me was ordering.  I took my sandwich and sat down at one of the tables, and it was then that I witnessed something truly inspiring.

The guy that I had treated decided to pay for the next lady's sandwich.

Who then, in turn, paid for the couple behind her.

This simple act of generosity was repeated six more times , the streak ending only when a customer ordered eight sandwiches for himself and his co-workers.  The guy whose "turn" it was looked at him and said with a chuckle, "Okay, I'm nice, but I'm not THAT nice."  It got a big laugh from the customers as well as the Subway employees (or, as they like to be called, "sandwich artists").

My point is this.  That series of exchanges certainly brightened my day, as I'm sure it did for everyone else who was there, and it reinforced the sometimes-dying belief that people are, by nature, good.  It's a lunch that I'll never forget.

Think about the exchanges we have with each other hundreds of times a day.  Saying "good morning" to a stranger on the street.  Flipping the bird to the asshole that cuts you off in traffic.  Reading your kids a bedtime story and tucking them in.  Some of these exchanges will be remembered forever, some forgotten almost immediately, but they're exchanges all the same.

The next time you walk in to the bank, order your morning coffee, take your seat on the bus, show up at work in the morning . . . you're going to interact with another person.  It's unavoidable.

What kind of evidence will you leave?

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